Fix build with newer CMake versions

We can no longer refer to targets in subdirectories in the TARGET
property of add_custom_command().

CMakeLists.txt

@@ -39,13 +39,14 @@ endif()
 
 add_subdirectory("arx")
 
+add_custom_target(copy-libs ALL)
 foreach(prefix IN LISTS CMAKE_PREFIX_PATH)
  set(libdir "${prefix}/bin/${CMAKE_LIBRARY_ARCHITECTURE}")
  if(IS_DIRECTORY "${libdir}")
  file(GLOB libs RELATIVE "${libdir}" *)
  foreach(lib IN LISTS libs)
  add_custom_command(
- TARGET arx POST_BUILD
+ TARGET copy-libs POST_BUILD
  COMMAND ${CMAKE_COMMAND} -E copy_if_different "${libdir}/${lib}" "$<TARGET_FILE_DIR:arx>/${lib}"
  )
  endforeach()